What Is the Key Flaw in Studies of the Effect

What is the key flaw in studies of the effect of exercise on mental abilities in which comparisons are made between regular exercisers and sedentary adults?

A) The exercisers use so many different forms of exercise that we can't be sure what works.
B) Sedentary subjects may actually exercise somewhat but not report it.
C) Those who choose to exercise may be different in other significant ways from those who choose to be sedentary.
D) The samples of exercisers are normally younger than the samples of sedentary adults to whom they are compared.